There is no one cart which is best for the 1200 bcoz we each have different associated gears and tastes. It's like cooking a dish. We both may have the basic fresh chicken, but depending on the ingredients on hand, we may end up cooking it a different way. It also depends on your taste. If you want a warm sound, you'd have to get a warm sounding cart. Bottomline, I wouldn't exceed the cost of the 1200 for the cart choice. Carts I've tried are Shure M44-7, Shure V15xMR(phased out), Grado Red, Grado Gold, Denon 103, AT 440mLA, etc carts. Take your pick. I mount each cart on a separate headshell and swap them when I tire of the sound. It's all about system matching. The best and easiest way to change the sound of your analog system is to change cartridges. Just make sure your phono can handle them. If you are using built-in phono of a receiver, then your stuck with MMs unless you use a step-up tranny. If you use a high output MC, then most outboard phonos with active MCs will work. Its when you want to use a low out put MC that the chioce of phono becomes limited and expensive. Besides, you won't mount an expensive low output MC on a 1200.
As to mounting a rega arm, You have more options for carts. Dynavector 10x5, Denon DL-103/R, Dyna 17D3/XX2? Choices increase. You will have to use a new armboard to mount the Rega arm like that made by Origin Live.
In my listening room, I have 5 ttables/6 tonearms set-up right now to play on the fly. Why? Because no one cart/tonearm/ttable combo is the best for ALL RECORDINGS. Sometimes you tire of how it sounds. Rather than keep swapping carts, just put the LP on another ttable and play it. I know, it's costly, but the ttables were acquired slowly over the years. Rather than let the idle in storage, I put them to use.